1. Field of the Invention
The present invention relates to a bent-forming method for forming a formed member such as a car part, and the like by bending a metal plate, and in particular, to prevention of shape defectives from occurring to the formed member, caused by elastic recovery after removal of the formed member from forming dies.
2. Description of the Related Art
In recent years, reinforcement as well as reduction in weight of a car body and its parts have been promoted in an attempt to attain objects of mileage improvement, environmental conservation, safety improvement, and so forth, and progress has been made in use of a lighter material, such as a steel sheet of high-tension, an aluminum alloy sheet, and so forth, for press-formed members of a metal sheet, accounting for the majority of constituent parts of the car body, as one of means for achieving the objects.
For example, FIG. 1A shows a Z-shaped member as a press-formed member, comprising an horizontal flat portion 1, a inclined flat portion 3 continuing from one end of the horizontal flat portion 1 via a first bend 2, and having one end thereof, extended downward, and a flange portion 5 coupled to the other end of the inclined flat portion 3 via a second bend 4, so as to be parallel with the horizontal flat portion 1, and FIG. 1B shows a hat-channel shaped member formed by disposing the Z-shaped members in bilateral symmetry.
A problem with press-forming of a metal sheet such as a high-tension steel sheet, and the like is that large elastic recovery (springback) occurs upon removing the metal sheet from a forming die, resulting in deterioration in dimensional accuracy of a formed member. This is shown in FIG. 2 by taking an example of the hat-channel shaped member. In the figure, a member shape (target shape) of the hat-channel shaped member before removal from the forming die is indicated by dash and double dashed lines while a member shape thereof after removal from the forming die is indicated by solid lines. An angle change defect Δθp (an angle formed between a tangent to a bend-stop face of the first bend, on a side thereof, adjacent to the inclined flat portion, and the inclined flat portion in a target formed shape) occurs to the first bend 2, and warpage curved outward (a maximum parting distance between the warpage and a line segment interconnecting bend-stop points at respective ends of the inclined flat portion is denoted by δ) occurs to the inclined flat portion 3 subjected to bending by bending-back. The bending by the bending-back refers to a phenomenon where after a sheet has once been formed by bending, a bent portion of the sheet is subjected to forming by bending, in a direction opposite to a direction of an initial forming by bending at the outset (bending-back), and a bent-back portion undergoes springback in the direction of the initial bending at the outset to be thereby deformed. Due to the angle change defect and warpage as described, an opening with an opening distance ΔD (a horizontal distance between the lower end of the inclined flat portion in the formed shape of the hat-channel shaped member after removal from the forming die, and that of the inclined flat portion in the target formed shape) occurs to the respective lower ends of the inclined flat portions 3 of the hat-channel shaped member. In this connection, the respective lower ends of the inclined flat portions of the hat-channel shaped member (the same applied to the Z-shaped member) refer to respective portions thereof, corresponding to an intersection of the inner face of the inclined flat portion, and an extension of the underside face of the flange portion.
As methods for preventing shape change defects from occurring to the formed member, there have been proposed, for example, a method of forming a reverse bending radius portion, oriented in a direction opposite from a bending direction for forming, along the ridge of a bend, and a method of providing compression stress in the direction of a sheet thickness at a bend, thereby reducing residual stress. Those methods, however, have problems of providing an originally unavailing shape, requiring special equipment, and so forth. Meanwhile, a forming method for improving shape defects without the problems described have been proposed in U.S. Pat. No. 6,748,788, and the 53rd Plastic Working Joint Conference Proceedings, pp. 251 to 252. As shown in FIG. 3, with this method, when a metal sheet P is formed by bending with a bending blade 52 descending toward a forming punch 51, the metal sheet P is bent in an arch-like shape along a bend forming portion 55 of the forming punch 51, and as the bending blade 52 further descends, portions of the surface of the metal sheet P, on a protruded side, are butted against an inclined flat forming portion 58 of the bending blade 52, and an inclined flat forming portion 56 of the forming punch 51, respectively, whereupon the metal sheet P is formed so as to undergo deformation such that arched parts A1, A2, having respective protuberances oriented in directions opposite from each other, are formed so as to be coupled with each other, thereby checking the angle change defect Δθp at the first bend 2, and the warp δ of the inclined flat portion 3 as described with reference to the Z-shaped member, and the hat-channel shaped member. Incidentally, a deformation behavior where the two arched parts A1, A2, having the respective protuberances oriented in directions opposite from each other, are formed so as to be coupled with each other is referred to as “dancing”.
With adoption of the forming method utilizing the dancing described, it has become possible to suppress the angle change defect Δθp occurring to the first bend 2, and the warpage δ occurring to the inclined flat portion 3 with reference to the Z-shaped member, and the hat-channel shaped member. However, in order to suppress such deformations as described, it has been necessary to form an bent angle θp of the bend forming portion 55 of the forming punch 51 so as to be an obtuse angle, posing therefore a problem that a formed shape is inevitably a particular shape in which the inclined flat portion 3 is inclined to the horizontal flat portion 1. Furthermore, there has arisen another problem that with reference to an angle θd formed between the inclined flat portion 3, and the flange portion 5, an angle change defect Δθd comes to be observed larger as the angle θd becomes closer to a right angle.